Interpretive Summary: The beef industry has responded to safety concerns surrounding E. coli O157:H7 in the ground beef by implementing a process called test-and-hold where products are not released into commerce until testing negative for the pathogen. The amount of enrichment media used to determine whether the pathogen is present is the most expensive part of the test-and-hold process. Recently, we reported that the ratio of enrichment media to sample (either ground beef or boneless beef trimmings) could be reduced from 10:1 down to 3:1 without affecting the ability to detect E. coli O157:H7. In the studies presented here, we tested ratios of enrichment media to sample from 3:1 down to 0 for their effect on detection of E. coli O157:H7 using culture isolation or commercial detection methods. The ratio of enrichment media to either beef trim or ground beef could be reduced to 1:1 without affecting the detection of E. coli O157:H7, regardless of the method used.

Technical Abstract:
The beef industry has responded to safety concerns surrounding Escherichia coli O157:H7 contamination in ground beef by implementing a process called test-and-hold. The test-and-hold process requires rapid and affordable test methods. Recent work from our laboratory showed tryptic soy broth (TSB) was a superior enrichment media for use in test-and-hold methods at levels down to a sample ratio of 3 volumes of media to 1 volume of sample. In the studies presented here, lower ratios of enrichment media to sample were examined for their effect on culture isolation, and molecular detection methods (BAX E. coli O157:H7 MP test, and Assurance E. coli O157:H7 GDS). Ground beef and boneless beef trim were inoculated with high level (170 CFU/65g ground beef and 43 CFU/65g trim) and a low level (17 CFU/65g ground beef and 4 CFU/65g trim) of E. coli O157:H7 and enriched in 3, 1, 0.5 and 0 volumes of TSB. Results indicated that the volume of enrichment media added to either beef trim or ground beef did not affect the culture isolation of E. coli O157:H7 from the samples. However significant differences in the levels of background bacterial growth were observed. The amount of culture enrichment media used did not affect the detection of E. coli O157:H7 in the ground beef or trim when the Assurance GDS system was used, or in ground beef when BAX MP was used. However, BAX-MP detection of E. coli O157:H7 in beef trim went from 50%, to 42% to 33% positive as enrichment volumes went from 0.5x, to 1x to 3x. Optimum results with all methods were observed using 1 volume of TSB to volume of sample. We conclude that E. coli O157:H7 detection test results can be considered valid if less than the specified 3 or 10 volumes of media are used.